1. Field of the Invention
This invention relates to the mining of subterranean solid sulfur by the Frasch method in which hot water is sent below the surface to liquefy the sulfur and the sulfur in liquid form is brought to the surface. In such a process, the hot water accumulates above at least a part of the subterranean solid sulfur. This invention more specifically relates to the conservation and utilization of the heat energy contained in the accumulated subterranean hot water.
2. Brief Description of the Prior Art
Sulfur occurs in the caprock of certain salt domes along the coastal area of the Gulf of Mexico and in the offshore waters of the Gulf. The salt domes are believed to have been formed by the intrusion of salt from extremely deep lying beds of salt into the sedimentary formation of this region. The salt intrusions are circular or elliptical in cross section and the tops of the domes vary in depth below mean sea level, usually from less than one hundred feet to several thousand feet. Occasionally, the tops rise to the surface. The tops of some salt domes are capped with limestone, anhydrite, gypsum, or a combination of these minerals. A cross section of a typical salt dome is shown in FIG. 1.
In the caprock of some of the domes of this type containing limestone with other minerals present, sulfur is found in fissures, cracks, seams and dispersed through the formation. Sulfur is also occasionally present to a lesser extent in the gypsum and anhydrite associated with the limestone of the dome. Most salt domes which are below the surface of the area in which they lie are covered with a layer of shale or other sediments which in essence forms the caprock into a closed container. The sulfur formation is often sandwiched between a layer of overlying barren limestone and an underlying layer of anhydrite. Underneath the anhydrite is the salt proper. Fewer than 10% of the salt domes discovered so far in the coastal region of the Gulf of Mexico have contained sulfur that is economically minable in commercial quantities.
After a dome has been discovered and has been proven to possess economically minable commercial quantities of sulfur in the caprock, the Frasch system of mining is usually initiated. In a typical system, a hole is drilled to a selected zone in the sulfur-bearing limestone by means of oil field type drilling equipment. The well, after drilling, usually is equipped with three concentric pipes within a protective casing which is cemented into the top of the caprock. Inside the outer casing a six-inch pipe is sunk through the caprock to the bottom of the sulfur deposit. The six-inch pipe is perforated with small holes in its lower end portion. Then a three-inch pipe is lowered to a point spaced a short distance from the bottom. Last, and innermost, is a one-inch pipe carrying compressed air and reaching more than half way to the bottom of the well.
Water, heated under pressure to about 325.degree. F. (well above the normal 212.degree. boiling point) is pumped down the space between the six-inch and three-inch pipes, and, during the initial heating period described above, also down the three-inch pipe. The initial heating period can extend for periods of 24 to 96 hours and water is injected during this period at the rate of 250 to 750 gallons per minute. The "superheated" water flows out of the holes at the bottom of the six-inch pipe into the sulfur bearing deposit and moves upwardly because of the lower density of the hot water compared to the colder connate water. As the temperature of the sulfur bearing formation reaches and exceeds the melting point of the sulfur, liquid sulfur flows to the bottom of the well, as it is approximately twice as heavy as water. The pumping of water down the three-inch pipe is then discontinued. Static pressure of the hot water forced into the formation, plus pressure imposed within the dome, then forces liquid sulfur several hundred feet up the three-inch pipe. Compressed air forced down the small pipe aerates and lightens the liquid sulfur in the three-inch pipe so that it will rise the rest of the way to the surface. A single well can take the sulfur from only about a half acre of dome area. So new wells must be drilled continually, and new pipelines laid to bring in water and air and carry off the molten sulfur. Other pipe sizes may be used, but this in no way changes the general theory of Frasch mining. Mining systems of the Frasch type are disclosed in U.S. Pat. Nos. 1,612,453 and 928,036.
Since the caprock of the salt dome is essentially a closed container, the injection of hot water from mining purposes will build up the pressure in the dome unless it is relieved. Relief is accomplished by drilling "bleedwells" to the floor of the dome and removing cold water. Cold water--as cold as practical--is removed to conserve heat in the dome and to maintain the desired mine pressure. In the course of time, large quantities of hot water accumulate in the upper regions of the dome in the barren areas and the leached areas (i.e., areas from which substantially all of the sulfur has been removed). The injection of millions of gallons of hot water (325.degree. F.) into the hydraulically closed domes, with the removal of cold water for pressure control, has resulted in the accumulation of very large quantities, e.g., up to trillions of BTU's, of heat within the domes. The fluid densities within the caprock are such that the hot water rises as it exits the well bottom, and percolates upwardly through the sulfur-rich limestone. Although the water is cooling as it gives up its heat to the melting sulfur and the surrounding formation, it is still very hot when it enters the formations above the sulfur ore. In fact, temperatures in the range of 220.degree. F. to 290.degree. F. are frequently measured in this spent water near the top of the caprock. From this description, it can be seen that, in a typical Frasch process mine, a large limestone caprock exists with the lower portions containing elemental sulfur enrichment within the limestone matrix and with the upper portions containing a vast induced geothermal resource of hot water accumulated from past and ongoing mining operations. A large group of wells can be drilled into the lower portion of the caprock with virgin hot water being injected to continue the sulfur melting process.
Water, returned to the earth's surface (hereinafter designated as bleedwater), is still at an elevated temperature, and in addition to the usual constituents of ground waters such as chlorides, sulfates and bicarbonates of sodium, calcium and magnesium, contains hydrogen sulfide, thiosulfates, hydrosulfides and polysulfides, and other sulfur compounds of various basic elements (which will be designated hereinafter simply as sulfides) and various other dissolved substances. The sulfides and the other constituents in the bleedwater render it highly corrosive and extremely destructive to the usual materials encountered in the commercial operation of a sulfur mine by the Frasch process.
The re-use of the hot bleedwater would require all conduits and equipment with which it came into contact to be made of special noncorrosive materials, which are extremely costly. Commercial recovery of the heat in bleedwater has been attempted using a closed type of heat exchanger in which the bleedwater flows over conduits carrying colder fresh water, the heat transfer being from the bleedwater through the conduit material and into the fresh water. Only a small part of the heat can be commercially recovered in this manner and the heat exchangers must be constructed of costly non-corrosive materials.
The fundamental problem in the re-use of hot bleedwater for mining is that not only the bleedwater reheating plant, but also the water distribution pipelines and production wells are subjected to severe corrosion and scaling. U.S. Pat. No. 2,109,611 illustrates one attempt to treat bleedwater to render it suitable for re-use. The corrosion and scaling are a particularly important problem in the Frasch process production wells since, in certain modes of operation, the concentric well pipelines would be contacted on both sides by the bleedwater. While corroded distribution pipelines on the surface are relatively easy to replace, corroded well pipelines could very easily render the entire well inoperable. U.S. Pat. No. 1,764,538 illustrates one attempt at bleedwater re-use in a standard sulfur mining well which would suffer from these difficulties.
It has, therefore, been customary to attempt to locate the bleedwells so as to return to the surface as cold a bleedwater as possible and to discharge this water to waste. This practice has accounted for great losses by the sulfur mining industry in the past because of the non-recovery of the heat from the bleedwater, by the inability to re-use the bleedwater and by loss of heat due to the flow of the hot mine water to the upper formations of the deposit where it is no longer available for melting sulfur in situ. The disposal of the bleedwater thus produced is subject to the further disadvantage that the suspended and dissolved matter, hydrogen sulfide and metal sulfides contaminate the surface water into which the bleedwater may be permitted to flow. In order to prevent objectionable pollution, then, it is necessary to purify the bleedwater before its discharge. The apparatus and process of purification before disposal impose a heavy expense upon the sulfur mining industry.
The re-use of bleedwater brought to the surface could provide tremendous economic and environmental benefits. Several efforts have been made in this direction. Excessive scaling and corrosion, however, have caused many of these operations to be uneconomical.
One of the more successful of these prior art attempts to utilize the bleedwater heat is set forth in U.S. patent application Ser. No. 819,879 now U.S. Pat. No. 4,157,847 to R. L. Williams et al. entitled "Method and Apparatus for Utilizing Accumulated Underground Water in the Mining of Subterranean Sulphur" filed on July 28, 1977 and assigned to the assignee of this application. The method and apparatus described in the Williams et al. application differs from the present system insofar as it does not bring the hot bleedwater to the surface, mix it with fresh hot water, and return the mixture to the sulfur deposit via a separate downpipe. Rather, it uses a jet pump device to achieve a subterranean recirculation of the hot bleedwater.
In some instances in the past sulfur wells have been pumped with their liners bleeding water to the atmosphere, creating concurrent flows of sulfur and water. Heat is supplied to such wells by injecting hot mine water down through the caprock casing. The very aggressive nature of the bleedwater renders this technique highly corrosive to the liner and the sulfur delivery pipe. In addition, a considerable quantity of heat is lost to the atmosphere and the large amounts of bleedwater at the surface present serious pollution problems.
U.S. Pat. Nos. 3,525,550; 3,432,205; and 3,258,069 describe methods and apparatuses which seek to take advantage of natural hot geopressured aquifers disposed below subterranean sulfur deposits and depend upon special geological formations that may be rare or difficult and expensive to locate. U.S. Pat. No. 3,432,205 circulates hot water down one well and through the sulfur-bearing formation to a second well, upwardly through which moves a mixture of sulfur and hot water and produces large amounts of corrosive water which must be disposed of. U.S. Pat. No. 3,630,573 describes an attempt to reduce the amount of hot water injected into a sulfur well by separately injecting superheated steam and hot water. U.S. Pat. No. 1,339,621 discloses an air lift specially designed in the shape of a Venturi to give rising molten sulfur an extra lift. The methods and apparatus of these patents, however, do not utilize the recycling of hot water to conserve and use the large amount of heat in the subterranean accumulated hot water resulting from subterranean sulfur mining operations.
U.S. Pat. No. 3,938,592 describes a method for extracting heat from subterranean rock strata which have been fractured by one or more explosions and filled with stratal fluid to absorb heat from the rock strata. The heated stratal fluid is then recycled upwardly to heat a heat-carrying agent which is recycled through a heat exchanger to a surface plant where the heat values may be utilized. U.S. Pat. No. 3,333,638 refers to the disposal of water from a gas-producing zone to a lower water-absorbing zone. U.S. Pat. No. 3,515,213 refers to the recovery of shale oil by circulating hot water from the surface through the shale and back to the surface again using two wells. U.S. Pat. Nos. 2,742,091; 2,871,948; 2,980,184 and 3,322,195 all relate to various treatments of oil wells to rejuvenate them and obtain additional production. U.S. Pat. No. 2,742,091 discloses a method in which hot oil is recycled within the well or casing. None of these patents disclose or suggest the mining of sulfur wherein the subterranean hot water accumulations from previous mining operations are recycled through a separate downpipe to heat underlying sulfur for liquefying it.